If it were contained in some way, that would make sense to me, but it's free to roam as far as I can see. As for the hay mat, that still seems strange to me, but fair enough.
So he looks litter trained based on the box, so my "guess" is it's to help keep any related stains to this carpet, same you don't wanna put you litter box on a carpet for a cat, also if all his toys/food are on the rug, he'll probably stay on the rug most of the time.
TIL you can litter train a bunny, really wish I knew that growing up since my family had a pet bunny when I was young and had to constantly clean up after it. I was wondering how anyone could do this with a bunny shitting all over the house.
I've got a couple guinea pigs. one assumes the tarp is the limit of his territory, whether or not there's fencing.
the other one regularly goes wandering, but respects the areas I've said are off limits. and she knows she has to get back to her territory - the tarp - to get any treats.
so it's entirely possible this bunny recognizes his "room", as defined by the new rug.